1. Field of the Invention
The present invention relates to a suspension control system for an automotive vehicle for controlling a damping coefficient of a shock absorber (also called damper, but hereinafter referred to as the shock absorber). The present invention, particularly, relates to the suspension control system described above in which each of the shock absorbers is provided with multiple variable damping coefficients stages.
2. Description of the Background Art
Japanese Utility Model Registration Application First Publication No. Showa 59-117510 published on Aug. 8, 1984 exemplifies a previously proposed vehicular suspension control system which carries out a rolling control for the vehicular body.
In the previously proposed vehicular suspension system, at least one of a damping force of the shock absorber and a spring constant of an pneumatic spring is increased on the basis of a signal derived from a lateral acceleration sensor which detects an applied acceleration in a vehicular width direction so as to hold the shock absorber in a hard damping force condition, thus suppressing vehicle roll.
However, since the suspension system described above maintains the shock absorber in the hard damping force condition during the rolling control, vibrations generated on an unsprung mass are transmitted to the vehicular body when the vehicle runs on a rough road during the rolling control so that vehicular comfort is deteriorated.
On the other hand, Japanese Patent Application First Publication No. Showa 64-60411 published on Mar. 7, 1989 exemplifies another previously proposed vehicular suspension control system.
In the latter previously proposed suspension control system, the damping force generated in the shock absorber is detected, a relative displacement between a sprung mass and unsprung mass is estimated according to the damping force signal intensity and the condition of the road on which the vehicle is running is determined according to a period of time at which the relative displacement exceeds a predetermined threshold value. When a flat or smooth road is determined, the shock absorber is controlled toward a low damping coefficient side during a predetermined interval of time. When a rough road is determined, the shock absorber is controlled toward a high damping coefficient side during the predetermined interval of time.
However, since in the latter previously proposed vehicular suspension control system the damping force control is carried out according to a signal (generated damping force of the shock absorber) mixed with the unsprung mass signal component and sprung mass signal component, a correct determination of whether it is time to control the vibration suppression or to absorb the vibrations so as to provide a preferable vehicular comfort cannot be made. Consequently, the vehicular ride comfort and steering stability are deteriorated.
In addition, since it requires a finite interval of Lime to determine the road surface condition, a time lag occurs in the damping coefficient control and thus causes a worsening of the vehicular ride comfort and steering stability.